Over recent years, there has been an increasing need for devices to not only detect the presence of radioactive sources, but also to establish, as simply and quickly as possible, the direction in which the detected radiation source is located.
Examples of such requirements include dismantling industrial plants in which radioactivity was used, where it is often necessary to inspect an (allegedly) cleared site which is to be released to establish whether all radioactive radiation sources have in fact been removed. If one or more radioactive sources have been left behind, it is of course desirable if these can be located as quickly as possible. This not only increases the speed of the clearance process, but also reduces the radiation exposure of the individuals concerned.
A further area of application relates to the emergency services or helpers (for example, fire brigades) in the event of accidents. In this case too, the aim is not only to detect the presence of radioactive sources, if present, as rapidly as possible, but also to determine their position as quickly as possible. In such applications, particularly rapid detection is often even more important than in the applications described at the outset. In particular, especially swift location of any radioactive sources that may possibly be present may prevent undesirable dispersion of the material in question. Furthermore, the time window for entering the scene of the accident is often particularly short if there is a fire, for example. Another area of application is the regulatory sector, such as in the work of safety authorities (often also referred to as Homeland Security). Rapid detection and identification of any radioactive sources that may be present is also particularly important in this case, as this enables routine screening to take place immediately afterwards.
A detector system was proposed in U.S. Pat. No. 7,994,482 B2 for determining the direction of a gamma and/or a neutron radiation source. A plurality of detector devices are used for this purpose, these being arranged in a cross formation, for example, so that they can measure a large angular range. In this particular device, it is proposed that two detector materials should be sandwiched together back-to-back (in the form of two cuboid detector sets which are in contact with each other via their largest surface) such that it is possible to establish the side from which the ionizing radiation originates from the counting ratio of the two superimposed detector materials. By arranging different detector devices (a pair of detectors, for example) in a cross formation, said pair being arranged at a 90° angle to one another, it is possible to break down the direction of the radiation source to an angle of 90°. Even though the arrangement proposed in the above patent is functional, it has a considerable disadvantage in that it is not possible to measure the direction accurately. A further disadvantage is that the counting rates of the superimposed detector elements must be significantly different from one another. In practice, this is only possible if defined, particularly dense scintillator materials are used or if a shielding layer is arranged between the two detector regions. Both of these solutions lead to disadvantages, such as, for example, restricted usability of scintillator materials or adding an extra weight to the arrangement, which often means that this is no longer suitable for portable applications.
U.S. Pat. No. 8,067,742 B2 describes a further device for determining the position (azimuth angle) of a gamma source. In the device described in this patent, two longitudinal scintillation crystals are arranged at an angle to one another. The light pulses generated by the scintillation crystals as a result of incident gamma radiation are amplified by photodetectors, which are arranged on one side of each scintillation crystal, and the resulting counting rates are evaluated. By comparing the counting rates of the two scintillation detectors (due to the different angular position relative to one another, these scintillation detectors have different counting rates as a function of their orientation relative to the gamma source), it is possible to deduce the direction of the gamma source. Additional scintillation detectors can also be used to further increase accuracy. The US patent specifies an accuracy of up to 5° for the position accuracy of the gamma source. One disadvantage of the set-up described in said patent is that the scintillation crystals used in this case display symmetry for angles offset (in other words, rotated) to the left or to the right (based on a minimum or maximum). As a result, orientation angles can admittedly be determined quite accurately; however, it is not possible to say whether the radiation source is located to the right or to the left (or in front of or behind and/or above or below) the measurement apparatus. This measurement characteristic is disadvantageous, and the resulting disadvantages can go so far as to make the device virtually unusable, at least for many applications. In any event, as a result of the disadvantages (at least with a certain statistical probability), this leads to a significant increase in the measurement time, as it is usually necessary to “simulate” the missing left-right resolution, for example, by other means, such as by increasing or reducing the total counting rate, for example, by moving closer to or further away from the radiation source in question.